Implement simple scaling policy forcluster scaling in and out

Users are recommended to use Complex ScalingPolicy to describe their requirement of autoscaling, including both trigger condition and adjustment. However, in some existed use cases, user may still want to define the trigger condition without Senlin's help, e.g. most Heat users are used to defining ceilometer alarms and AutoscalingGroup respectively. So we may need to provide *simple* version ScalingPolicy to meet this demand.